.
Calculated by Natsink, assuming uniform temperatrure at the back surface, including radiation.
And it is for one single heatsink.
I think enough even for 2 channels of some small Class A power amp.
Don't you ?

We now have a price and a production lot.
We can keep to the 2500 HKD promised in post #1.
Schedule delivery forecast to be April 2019.
We'll have to ask for 50% down payment first.
Shall contact you by PM this weekend.

Also checked air parcel (with tracking) world wide.
Approx. 450 HKD for 3kg.
Exact weight not known yet.
And surface parcel does not save you 100 HKD.

I re-check the weight of my finished DAO HP (same case).
It comes to 12kg, including a 120VA toroidal transformer of 2kg.
So I was way out and we need to plane for 12kg shipment wieght.

That would mean 1100 HKD for air parcel, and 600 HKD for surface parcel.
I would propose to use surface parcel.
You of course always have the right to retract from the GB.
But if it less than 4 subscribers then we shall not go ahead.
Which is also less work for us.